This invention relates to a method and apparatus for highly accelerated life testing of photovoltaic solar cells.
Highly concentrated photovoltaics (HCPV) is an emerging solar technology with tremendous market potential because of its very high energy production density, its ability to scale to any size deployment, and its ability to operate more efficiently than other photovoltaic (PV) technologies in areas with high insolation and rapidly varying ambient and localized temperatures.
The success of HCPV in utility and commercial markets is closely linked to the ability of these systems to achieve grid parity with fossil fuel energy production. Elements in achieving a levelized cost of energy consistent with grid parity are decreased module cost, high reliability, and low operational and maintenance cost. Improved cell efficiency and reliability have a large impact in lowering all of these cost elements. Higher efficiency and reliability result in a lower module cost per watt since fewer modules are required to produce the initial target power and the power degradation is low. Maintenance costs are also reduced because there are fewer modules to maintain and the lower frequency at which they must be serviced due to cell degradation. Any advantages from higher cell efficiency, however, can be negated if the cell power output decreases rapidly over time. Likewise, a very small decrease in cell performance over time has little impact if the initial cell efficiency is low. Thus, it is important for the cell to have both high efficiency and high reliability.
One way to ensure high efficiency and high reliability is to use “Highly Accelerated Life Tests” (HALT) during solar cell development. HALT testing is designed to test semiconductor cells beyond expected field conditions to identify the operating and failure limits of the cell. Although HALT may also be used to determine solar cell lifetimes, the main goal of HALT is to identify, analyze, and correct inherent design weaknesses that may exist in the epitaxial, in the wafer fabrication processes and in the manufacturing/assembly process. The approach is to test devices to failure using HALT methodology with various stresses, to perform root cause analysis of the failure, and then to take corrective action to improve the process and/or product. The operating range and failure limits identified during HALT can also be used to develop a “Highly Accelerated Stress Screen” (HASS) for later phases of manufacturing or packaging processes, which will allow rapid identification of downstream process flaws.
Performing HALT on solar cells under real-life field conditions has proven difficult because of the rapid thermal transitions experienced by solar cells during their expected 30+ year lifespan. The known prior relevant art does not disclose how to adequately maintain the strict environmental controls necessary for rapid thermal transition stresses during longer term HALT on solar cells.
U.S. Patent Publication 2010/0046575 A1 discloses a method and system for performing thermal stress testing of photovoltaic solar cells. The system and method present a means of rapidly applying thermal stresses to one solar cell at a time under uncontrolled ambient conditions. The publication does not disclose an apparatus or method for HALT, nor does it disclose how to repeatedly and reliably use rate of temperature change as a stressor. It also does not disclose any method or apparatus to control the environmental conditions, which is particularly important when going to high and low temperatures.
U.S. Pat. No. 6,154,034 discloses a method and apparatus for testing multi junction photovoltaic solar cells using multiple flashlamps, with a separate flashlamp provided for each junction of the solar cell so that the spectrum of each flashlamp can be tailored to the spectrum in which the corresponding junction is designed to operate. This patent does not disclose any apparatus or method for suitable for HALT. It merely describes a flash tester for characterization and does not disclose or teach any methods for accelerated life testing.
US Patent Publication 2002/0171441 A1 discloses a method and apparatus for accelerated life testing (ALT) of solar cells in which high-intensity laser light (generated by a diode) is focused onto a small area of a solar cell for a predetermined period of time to simulate long-term sun exposure. Performance of the solar cell is measured to determine life expectancy. The application uses diode laser light to overcome the heat build-up and focusing difficulties created by Xenon discharge light. The application does not disclose an apparatus or method for HALT. It also does not disclose any method or apparatus to control the environmental conditions, which is particularly important when going to high and low temperatures.
What is needed is an apparatus and method for performing HALT that accelerates the rapid thermal transition stresses as well as atmospheric stresses in a controlled environment on a solar cell in a short timeframe beyond the level of environmental stresses that a cell might encounter in its 30+ years lifetime.